Remote oceanic islands provide unique scenarios in which to study the factors involved in population divergence and the formation of new species. The Reunion gray white-eye (Zosterops borbonicus) shows a striking, geographically structured plumage polymorphism on the topographically and ecologically complex yet small island of Reunion. In contrast, its sister species (Z. mauritianus) is monomorphic on the relatively uniform island of Mauritius. In collaboration with Christophe Thébaud (http://thebaud.weebly.com/) at the University of Toulouse, we are using various genetic, genomic and phenotypic analyses to investigate the evolutionary history of this unique intra-island radiation. We are specifically interested in the relative roles of vicariant barriers (mountains, rivers), ecological factors (altitudinal and ecological gradients) and sexual selection, in driving the process of divergence among populations. Our work is funded in part by France’s Agence Nationale de la Recherche (ANR) and National Geographic Society.

To date, our data for Reunion reveal marked genetic structure across the island, apparently driven by selection acting on very specific regions of the genome. A preliminary tree based on 20,000 SNP (single-nucleotide polymorphism) loci obtained by RADseq (restriction-site associated DNA sequencing), reveals that Z. borbonicus is divided into two main clades corresponding to ecologically different highlands and lowlands. In turn, the three lowland morphs (lowland brown, gray-headed brown, and gray-headed brown-naped) also form independent genetic lineages within the lowland clade.

Thus our genetic and morphometric results for Reunion show the early role of ecology in dividing populations into highland and lowland forms, and the existence of independent lineages in the lowlands suggests the existence of premating isolating barriers to reproduction, perhaps due to sexual selection on plumage traits. Despite being good flyers, our results indicate that dispersal in the gray white-eye is extremely limited and suggest the role of selection in restricting gene flow between incipient evolutionary lineages.